summaryrefslogtreecommitdiffstats
path: root/src/runtime/mbarrier.go
blob: 46ef42f74dd5d34606b369bdc6dfea808d887652 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Garbage collector: write barriers.
//
// For the concurrent garbage collector, the Go compiler implements
// updates to pointer-valued fields that may be in heap objects by
// emitting calls to write barriers. The main write barrier for
// individual pointer writes is gcWriteBarrier and is implemented in
// assembly. This file contains write barrier entry points for bulk
// operations. See also mwbbuf.go.

package runtime

import (
	"internal/abi"
	"internal/goarch"
	"unsafe"
)

// Go uses a hybrid barrier that combines a Yuasa-style deletion
// barrier—which shades the object whose reference is being
// overwritten—with Dijkstra insertion barrier—which shades the object
// whose reference is being written. The insertion part of the barrier
// is necessary while the calling goroutine's stack is grey. In
// pseudocode, the barrier is:
//
//     writePointer(slot, ptr):
//         shade(*slot)
//         if current stack is grey:
//             shade(ptr)
//         *slot = ptr
//
// slot is the destination in Go code.
// ptr is the value that goes into the slot in Go code.
//
// Shade indicates that it has seen a white pointer by adding the referent
// to wbuf as well as marking it.
//
// The two shades and the condition work together to prevent a mutator
// from hiding an object from the garbage collector:
//
// 1. shade(*slot) prevents a mutator from hiding an object by moving
// the sole pointer to it from the heap to its stack. If it attempts
// to unlink an object from the heap, this will shade it.
//
// 2. shade(ptr) prevents a mutator from hiding an object by moving
// the sole pointer to it from its stack into a black object in the
// heap. If it attempts to install the pointer into a black object,
// this will shade it.
//
// 3. Once a goroutine's stack is black, the shade(ptr) becomes
// unnecessary. shade(ptr) prevents hiding an object by moving it from
// the stack to the heap, but this requires first having a pointer
// hidden on the stack. Immediately after a stack is scanned, it only
// points to shaded objects, so it's not hiding anything, and the
// shade(*slot) prevents it from hiding any other pointers on its
// stack.
//
// For a detailed description of this barrier and proof of
// correctness, see https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md
//
//
//
// Dealing with memory ordering:
//
// Both the Yuasa and Dijkstra barriers can be made conditional on the
// color of the object containing the slot. We chose not to make these
// conditional because the cost of ensuring that the object holding
// the slot doesn't concurrently change color without the mutator
// noticing seems prohibitive.
//
// Consider the following example where the mutator writes into
// a slot and then loads the slot's mark bit while the GC thread
// writes to the slot's mark bit and then as part of scanning reads
// the slot.
//
// Initially both [slot] and [slotmark] are 0 (nil)
// Mutator thread          GC thread
// st [slot], ptr          st [slotmark], 1
//
// ld r1, [slotmark]       ld r2, [slot]
//
// Without an expensive memory barrier between the st and the ld, the final
// result on most HW (including 386/amd64) can be r1==r2==0. This is a classic
// example of what can happen when loads are allowed to be reordered with older
// stores (avoiding such reorderings lies at the heart of the classic
// Peterson/Dekker algorithms for mutual exclusion). Rather than require memory
// barriers, which will slow down both the mutator and the GC, we always grey
// the ptr object regardless of the slot's color.
//
// Another place where we intentionally omit memory barriers is when
// accessing mheap_.arena_used to check if a pointer points into the
// heap. On relaxed memory machines, it's possible for a mutator to
// extend the size of the heap by updating arena_used, allocate an
// object from this new region, and publish a pointer to that object,
// but for tracing running on another processor to observe the pointer
// but use the old value of arena_used. In this case, tracing will not
// mark the object, even though it's reachable. However, the mutator
// is guaranteed to execute a write barrier when it publishes the
// pointer, so it will take care of marking the object. A general
// consequence of this is that the garbage collector may cache the
// value of mheap_.arena_used. (See issue #9984.)
//
//
// Stack writes:
//
// The compiler omits write barriers for writes to the current frame,
// but if a stack pointer has been passed down the call stack, the
// compiler will generate a write barrier for writes through that
// pointer (because it doesn't know it's not a heap pointer).
//
// One might be tempted to ignore the write barrier if slot points
// into to the stack. Don't do it! Mark termination only re-scans
// frames that have potentially been active since the concurrent scan,
// so it depends on write barriers to track changes to pointers in
// stack frames that have not been active.
//
//
// Global writes:
//
// The Go garbage collector requires write barriers when heap pointers
// are stored in globals. Many garbage collectors ignore writes to
// globals and instead pick up global -> heap pointers during
// termination. This increases pause time, so we instead rely on write
// barriers for writes to globals so that we don't have to rescan
// global during mark termination.
//
//
// Publication ordering:
//
// The write barrier is *pre-publication*, meaning that the write
// barrier happens prior to the *slot = ptr write that may make ptr
// reachable by some goroutine that currently cannot reach it.
//
//
// Signal handler pointer writes:
//
// In general, the signal handler cannot safely invoke the write
// barrier because it may run without a P or even during the write
// barrier.
//
// There is exactly one exception: profbuf.go omits a barrier during
// signal handler profile logging. That's safe only because of the
// deletion barrier. See profbuf.go for a detailed argument. If we
// remove the deletion barrier, we'll have to work out a new way to
// handle the profile logging.

// typedmemmove copies a value of type typ to dst from src.
// Must be nosplit, see #16026.
//
// TODO: Perfect for go:nosplitrec since we can't have a safe point
// anywhere in the bulk barrier or memmove.
//
//go:nosplit
func typedmemmove(typ *_type, dst, src unsafe.Pointer) {
	if dst == src {
		return
	}
	if writeBarrier.needed && typ.ptrdata != 0 {
		bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.ptrdata)
	}
	// There's a race here: if some other goroutine can write to
	// src, it may change some pointer in src after we've
	// performed the write barrier but before we perform the
	// memory copy. This safe because the write performed by that
	// other goroutine must also be accompanied by a write
	// barrier, so at worst we've unnecessarily greyed the old
	// pointer that was in src.
	memmove(dst, src, typ.size)
	if writeBarrier.cgo {
		cgoCheckMemmove(typ, dst, src, 0, typ.size)
	}
}

//go:linkname reflect_typedmemmove reflect.typedmemmove
func reflect_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
	if raceenabled {
		raceWriteObjectPC(typ, dst, getcallerpc(), abi.FuncPCABIInternal(reflect_typedmemmove))
		raceReadObjectPC(typ, src, getcallerpc(), abi.FuncPCABIInternal(reflect_typedmemmove))
	}
	if msanenabled {
		msanwrite(dst, typ.size)
		msanread(src, typ.size)
	}
	if asanenabled {
		asanwrite(dst, typ.size)
		asanread(src, typ.size)
	}
	typedmemmove(typ, dst, src)
}

//go:linkname reflectlite_typedmemmove internal/reflectlite.typedmemmove
func reflectlite_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
	reflect_typedmemmove(typ, dst, src)
}

// reflect_typedmemmovepartial is like typedmemmove but assumes that
// dst and src point off bytes into the value and only copies size bytes.
// off must be a multiple of goarch.PtrSize.
//
//go:linkname reflect_typedmemmovepartial reflect.typedmemmovepartial
func reflect_typedmemmovepartial(typ *_type, dst, src unsafe.Pointer, off, size uintptr) {
	if writeBarrier.needed && typ.ptrdata > off && size >= goarch.PtrSize {
		if off&(goarch.PtrSize-1) != 0 {
			panic("reflect: internal error: misaligned offset")
		}
		pwsize := alignDown(size, goarch.PtrSize)
		if poff := typ.ptrdata - off; pwsize > poff {
			pwsize = poff
		}
		bulkBarrierPreWrite(uintptr(dst), uintptr(src), pwsize)
	}

	memmove(dst, src, size)
	if writeBarrier.cgo {
		cgoCheckMemmove(typ, dst, src, off, size)
	}
}

// reflectcallmove is invoked by reflectcall to copy the return values
// out of the stack and into the heap, invoking the necessary write
// barriers. dst, src, and size describe the return value area to
// copy. typ describes the entire frame (not just the return values).
// typ may be nil, which indicates write barriers are not needed.
//
// It must be nosplit and must only call nosplit functions because the
// stack map of reflectcall is wrong.
//
//go:nosplit
func reflectcallmove(typ *_type, dst, src unsafe.Pointer, size uintptr, regs *abi.RegArgs) {
	if writeBarrier.needed && typ != nil && typ.ptrdata != 0 && size >= goarch.PtrSize {
		bulkBarrierPreWrite(uintptr(dst), uintptr(src), size)
	}
	memmove(dst, src, size)

	// Move pointers returned in registers to a place where the GC can see them.
	for i := range regs.Ints {
		if regs.ReturnIsPtr.Get(i) {
			regs.Ptrs[i] = unsafe.Pointer(regs.Ints[i])
		}
	}
}

//go:nosplit
func typedslicecopy(typ *_type, dstPtr unsafe.Pointer, dstLen int, srcPtr unsafe.Pointer, srcLen int) int {
	n := dstLen
	if n > srcLen {
		n = srcLen
	}
	if n == 0 {
		return 0
	}

	// The compiler emits calls to typedslicecopy before
	// instrumentation runs, so unlike the other copying and
	// assignment operations, it's not instrumented in the calling
	// code and needs its own instrumentation.
	if raceenabled {
		callerpc := getcallerpc()
		pc := abi.FuncPCABIInternal(slicecopy)
		racewriterangepc(dstPtr, uintptr(n)*typ.size, callerpc, pc)
		racereadrangepc(srcPtr, uintptr(n)*typ.size, callerpc, pc)
	}
	if msanenabled {
		msanwrite(dstPtr, uintptr(n)*typ.size)
		msanread(srcPtr, uintptr(n)*typ.size)
	}
	if asanenabled {
		asanwrite(dstPtr, uintptr(n)*typ.size)
		asanread(srcPtr, uintptr(n)*typ.size)
	}

	if writeBarrier.cgo {
		cgoCheckSliceCopy(typ, dstPtr, srcPtr, n)
	}

	if dstPtr == srcPtr {
		return n
	}

	// Note: No point in checking typ.ptrdata here:
	// compiler only emits calls to typedslicecopy for types with pointers,
	// and growslice and reflect_typedslicecopy check for pointers
	// before calling typedslicecopy.
	size := uintptr(n) * typ.size
	if writeBarrier.needed {
		pwsize := size - typ.size + typ.ptrdata
		bulkBarrierPreWrite(uintptr(dstPtr), uintptr(srcPtr), pwsize)
	}
	// See typedmemmove for a discussion of the race between the
	// barrier and memmove.
	memmove(dstPtr, srcPtr, size)
	return n
}

//go:linkname reflect_typedslicecopy reflect.typedslicecopy
func reflect_typedslicecopy(elemType *_type, dst, src slice) int {
	if elemType.ptrdata == 0 {
		return slicecopy(dst.array, dst.len, src.array, src.len, elemType.size)
	}
	return typedslicecopy(elemType, dst.array, dst.len, src.array, src.len)
}

// typedmemclr clears the typed memory at ptr with type typ. The
// memory at ptr must already be initialized (and hence in type-safe
// state). If the memory is being initialized for the first time, see
// memclrNoHeapPointers.
//
// If the caller knows that typ has pointers, it can alternatively
// call memclrHasPointers.
//
// TODO: A "go:nosplitrec" annotation would be perfect for this.
//
//go:nosplit
func typedmemclr(typ *_type, ptr unsafe.Pointer) {
	if writeBarrier.needed && typ.ptrdata != 0 {
		bulkBarrierPreWrite(uintptr(ptr), 0, typ.ptrdata)
	}
	memclrNoHeapPointers(ptr, typ.size)
}

//go:linkname reflect_typedmemclr reflect.typedmemclr
func reflect_typedmemclr(typ *_type, ptr unsafe.Pointer) {
	typedmemclr(typ, ptr)
}

//go:linkname reflect_typedmemclrpartial reflect.typedmemclrpartial
func reflect_typedmemclrpartial(typ *_type, ptr unsafe.Pointer, off, size uintptr) {
	if writeBarrier.needed && typ.ptrdata != 0 {
		bulkBarrierPreWrite(uintptr(ptr), 0, size)
	}
	memclrNoHeapPointers(ptr, size)
}

// memclrHasPointers clears n bytes of typed memory starting at ptr.
// The caller must ensure that the type of the object at ptr has
// pointers, usually by checking typ.ptrdata. However, ptr
// does not have to point to the start of the allocation.
//
//go:nosplit
func memclrHasPointers(ptr unsafe.Pointer, n uintptr) {
	bulkBarrierPreWrite(uintptr(ptr), 0, n)
	memclrNoHeapPointers(ptr, n)
}